This invention relates to fuel compositions of low sulphur content which contain at least one component capable of reducing particulate emissions from the exhausts of engines which generate power by combustion of such fuels.
Of particular interest are fuels such as diesel which are used widely in automotive transport and for providing power for heavy duty equipment due to their high fuel economy. However, one of the problems when such fuels are burned in internal combustion engines is the pollutants in the exhaust gases that are emitted into the environment. For instance, some of the most common pollutants in diesel exhausts are nitric oxide and nitrogen dioxide (hereafter abbreviated as xe2x80x9cNOxxe2x80x9d), hydrocarbons and sulphur dioxide, and to a lesser extent carbon monoxide. In addition, diesel powered engines also generate a significant amount of particulate emissions which include inter alia soot, adsorbed hydrocarbons and sulphates, which are usually formed due to the incomplete combustion of the fuel and are hence the cause of dense black smoke emitted by such engines through the exhaust. The oxides of sulphur have recently been reduced considerably by refining the fuel, e.g., by hydrodesulphurisation thereby reducing the sulphur levels in the fuel itself and hence in the exhaust emissions. However, the presence of particulate matter in such exhaust emission has been a more complex problem. It is known that the primary cause of the particulate matter emission is incomplete combustion of the fuel and to this end attempts have been made to introduce into the fuel organic compounds which have oxygen value therein (hereafter referred to as xe2x80x9coxygenatesxe2x80x9d) to facilitate combustion. Oxygenates are known to facilitate the combustion of fuel to reduce the particulate matter and the use of alcohols as oxygenates has been described in the prior art especially with respect to conventional diesel fuels which have a relatively high sulphur content of, e.g.,  greater than 200 ppm. For instance, U.S. Pat. No. 5,425,790 describes the use of alcohols and glycols for reducing particulate emissions from such relatively high sulphur diesel fuels. The authors confirm that the amount of reduction in particulate matter scales roughly linearly with the oxygen content of the component added although ethers seem to be more effective for reducing particulates than alcohols for the same oxygen content.
U.S. Pat. No. 4,378,973 discloses the use of a combination of cyclohexane and an oxygenated additive for reducing particulate emissions from fuels. This document states that the beneficial effect cannot be achieved in the absence of cyclohexane. This document discloses 2-ethyl hexanol and xe2x80x9cEPAL 1012xe2x80x9d which comprises a mixture of normal C6-C20 alcohols as the oxygenated additives. However, there is no mention of the sulphur content of such fuels.
A further reference, WO 93/24593, is primarily concerned with gasohol blends from diesel and alcohols. This blend must contain 20-70% by volume of ethanol or methanol, 1-15% by volume of a tertiary alkyl peroxide and 4.5-5.5% by volume of a higher straight chain alcohol. The straight chain alcohols disclosed have from 3-12 carbon atoms. According to this reference the presence of a tertiary alkyl peroxide is essential for the performance of the fuel since using 10% v/v alcohol performs no better than a straight diesel whereas 30% v/v of ethanol xe2x80x9cseverely degraded the engine""s operationxe2x80x9d (page 8, lines 14-19).
WO 98/35000 relates to lubricity enhancing agents and makes no mention of controlling or reducing emission of particulate matter. This document discloses the use of primary, linear C7+ alcohols in an amount of  less than 5% w/w of a diesel fuel composition.
U.S. Pat. Nos. 5,324,335 and 5,645,613 both in the name of the same assignee relate to fuels produced by the Fischer-Tropsch process which also contain inter alia alcohols formed in situ in the process which is recycled to the process. Whilst several primary alcohols are disclosed most of these are linear except the reference to methyl butanol and methyl pentanol. However, the streams recycled contain a considerable amount of other components such as, e.g., aldehydes, ketones, aromatics, olefins, etc. Also, the amount of alcohols generated by this process, especially the content of branched alcohols ( less than 0.5%), appears to be very low in relation to the total stream recycled. These two do refer to the use of Fischer Tropsch diesel fuels which have a sulphur content of less than 50 ppm.
U.S. Pat. No. 5,720,784 refers to fuel blends and the difficulty in rendering diesel fuels miscible with the conventionally used methanol and ethanol. This document purports to mitigate the problem of miscibility by adding to such formulations a C3 (excluding n-propanol)-C22 organic alcohol. However, whilst the document refers to the use of higher alcohols to form single phase compositions which are not prone to separation, it is silent on the nature of the diesel fuelxe2x80x94for these can vary significantly in their composition from light naphtha to heavy duty diesel oilsxe2x80x94nor indeed the effect of any of the alcohols referred to on the problems of particulate emissions when using such fuels in diesel fuel powered internal combustion engines. Furthermore, when addressing the issue of miscibility, it fails to distinguish between fuel compositions which contain the lower C1 and C2 alcohols and compositions which contain no lower alcohols. There is no mention of the sulphur content of fuels.
More recently, ashless diesel fuels having an ultra-low sulphur (xe2x89xa650 ppm) content are also known as Ultra Low Sulphur Automotive Diesel Oil (hereafter xe2x80x9cULSADOxe2x80x9d), a density of no more than 835 kg/m3, and a T95 (i.e., a temperature by which 95% of the fuel has distilled) of no more than 345xc2x0 C. have been developed. Such fuels are considered as xe2x80x9ccleanxe2x80x9d diesel fuels and are expected to have lower particulate emissions over a broad range of vehicles than the fuels of relatively higher sulphur content used hitherto.
WO 92/20761 discloses compositions comprising biodiesel in which the base fuels are predominantly esters and alcohols. There is no mention in this document of reducing particulate matter from emissions.